History · Engineering · 16 min read

The Titans of the Ghats: Engineering Marvels and Folklore of Idukki's Dams

Arch dams that join two legendary hills, a 130-year-old masonry wall that turned a river eastward, a tunnel bored through a mountain, and quiet reservoirs among the tea. The engineering and the stories behind Idukki's great dams.

By GK Global, Adimali, Munnar · Trails & Tales by GK

Idukki's rivers fall fast and far. Fed by two monsoons and rising on the highest ground in peninsular India, the Periyar and its tributaries drop more than two thousand metres from the grasslands of the High Range to the plains, carving deep gorges through ancient granite. For more than a century, engineers have looked at those falling waters and seen power, irrigation and prosperity. The structures they built here, arch dams and masonry walls, tunnels and underground power stations, rank among the boldest works of civil engineering in India. And around each of them, the people of the hills have woven stories that are older, and in some ways more enduring, than the concrete.

The Idukki Arch Dam Complex

Where two hills were joined

Deep in the Idukki forest, the Periyar squeezes through a narrow gorge between two great granite hills, Kuravan Mala (about 839 m) and Kurathi Mala (about 925 m). The Ooralis, the tribal people of these forests, have long told how the hills came to be.

A young Oorali couple, Kuravan and his bride Kurathi, lived in the forest. One day Rama, wandering in search of Sita, came to their hut, weary and thirsty. The newly-weds, lost in each other, did not see him and offered him nothing. In his grief and anger, Rama cursed them to become two hills, standing for ever apart, with the river between them. When they begged his forgiveness, he could not undo the curse, but he granted them a boon: one day, the hands of men would bring them together again.

In 1922, the Oorali headman Chemban Karuvellayan Kolumban guided two hunters through the forest: W. J. John, superintendent of the Malankara estate, and his companion A. C. Thomas. Kolumban led them to the gorge, told them the story of the two hills, and they saw at once that the narrow gap between Kuravan and Kurathi was a natural site for a dam. In 1932, John submitted a report to the Travancore government proposing a hydroelectric dam at Idukki. The project waited four decades, but when the great arch finally rose between the two hills, the Ooralis said that Rama's boon had come true. Kolumban is remembered with a memorial near the dam.

The arch

The Idukki Arch Dam is a double-curvature parabolic thin arch dam, curved both horizontally and vertically like the shell of an egg. Instead of resisting the water by sheer weight, as a gravity dam does, an arch dam transfers the thrust of the reservoir sideways into the rock of the valley walls. The granite of Kuravan and Kurathi, solid and close together, made this possible, and the result is a structure of astonishing slenderness for its size: barely 20 m thick at its base and less than 8 m at its crest, yet holding back one of the largest reservoirs in Kerala.

• Type: concrete double-curvature parabolic thin arch dam, one of the highest arch dams in Asia.
• Height: 168.91 m (554 ft) from the deepest foundation.
• Crest length: 365.85 m.
• Thickness: 19.81 m at the base; 7.62 m at the crest.
• Concrete: about 450,000 to 464,000 m³.
• Construction: sanctioned in 1963; main work from 1969; the dam was completed in the early 1970s.
• Builders: Hindustan Construction Company, with technical advice from the Canadian firm SNC (now SNC-Lavalin) and loans and grants from the Government of Canada.

Cheruthoni and Kulamavu

An arch dam alone could not hold the reservoir, because the water would escape through other low points in the hills. Two companion dams close the gaps. Cheruthoni, a concrete gravity dam about 138 m high on the Cheruthoni river, carries the spillway gates of the whole system; their rare openings, in 1981 and 1992, dramatically during the floods of 2018, and in some later monsoons, draw crowds and headlines across Kerala. Kulamavu, a masonry gravity dam about 100 m high, seals the reservoir on its western side. Together the three dams impound a reservoir of about 60 km², with a gross storage of nearly 2,000 million m³, spreading through the forested valleys of the Idukki Wildlife Sanctuary.

Moolamattom: a power station inside a mountain

The purpose of all this is electricity. From the reservoir, a tunnel and pressure shafts carry the water through the hills and drop it about 670 m to the Moolamattom power station, built in a vast cavern deep inside the Nadukani hill, one of the largest underground power stations in India. There, six Pelton turbines of 130 MW each give an installed capacity of 780 MW, making it Kerala's largest hydroelectric station. The first machine ran in October 1975, and Prime Minister Indira Gandhi dedicated the project to the nation in February 1976. After passing the turbines, the water flows out into the Thodupuzha river on the plains, by a different route from the one nature intended.

• Power station: underground, at Moolamattom.
• Installed capacity: 780 MW (6 × 130 MW).
• Gross head: about 669 m.
• Commissioned: 1975–76 (first stage), with later units added.

The dams are open to visitors only at set times, usually during holiday seasons such as Onam and Christmas, and boat trips run on the reservoir. Kalvari Mount, nearby, offers a view of the reservoir all year round. (Read Kalvari Mount.)

Mullaperiyar: the dam that turned a river

A river sent east

Every river in central Kerala runs west, to the Arabian Sea. But on the far side of the mountains lay the dry plains of Madurai and Ramanathapuram, parched in the rain shadow of the Ghats, where famine was a regular visitor. As early as the 18th century, the rulers of Ramnad dreamt of diverting the waters of the Periyar across the watershed. In the 1880s the Madras government took up the idea. Under a 999-year lease signed with the Maharaja of Travancore on 29 October 1886, it acquired the right to dam the Periyar, high in the forests of what is now the Periyar Tiger Reserve, and send its waters east through a tunnel into the Vaigai basin.

• Type: masonry gravity dam, built of limestone and surkhi (burnt brick powder, used as a hydraulic mortar), faced with rubble masonry.
• Height: about 53.6 m (176 ft) above the foundation.
• Length: about 366 m (1,200 ft).
• Construction: 1887–1895; opened on 10 October 1895.
• Diversion: a tunnel about 1.7 km long carries the water through the ridge into the Vaigai system, irrigating the districts of Theni, Dindigul, Madurai, Sivaganga and Ramanathapuram.

Pennycuick and the builders

The man in charge was Colonel John Pennycuick (1841–1911) of the Madras Engineers. The work was brutal: dense forest, heavy monsoons, wild animals, malaria and cholera. Twice, floods swept away the coffer dams that held back the river while the foundations were laid, and the government grew reluctant to spend more. A celebrated story says that Pennycuick then went home to England and sold his family property to keep the project alive; other versions say his wife sold her jewellery. These stories are told everywhere in the Cumbum valley, but historians have found no documents to prove them, and they are best seen as tradition. What is certain is that Pennycuick pursued the project with extraordinary determination, and that the dam was completed, against great odds, in 1895.

The cost in human life was terrible. Thousands of labourers, many of them from the plains of Tamil Nadu, worked on the dam, and British records suggest that many hundreds, perhaps far more, died, above all of cholera. Their names are mostly forgotten.

In the villages of Theni, families still cook pongal before Pennycuick's portrait at harvest time, and children are named after him.

Today the old dam is the subject of a long dispute between Kerala, which fears for the safety of a 130-year-old structure upstream of millions of people, and Tamil Nadu, whose farms depend on its water. (Read the full story in The Mullaperiyar dam, and see where its water goes in the vineyards of Cumbum.)

The Anchuruli Tunnel

When the engineers planned the Idukki project, they wanted every drop of water they could find. Near Kattappana, the Erattayar river flowed the wrong way, away from the reservoir. So they built a small dam on the Erattayar and bored a tunnel straight through the hill of Kalyanathandu to bring its water into the Idukki reservoir.

• Length: 3.77 km (2.34 miles), according to the Kerala State Electricity Board's specifications for the Erattayar diversion (popular sources often say 5.5 km).
• Section: circular, about 7.3 m (24 ft) in diameter.
• Construction: March 1974 to January 1980, by the contractor Pailey Pillai of Kolenchery, for the KSEB.
• Cost in lives: about 22 workers died during construction.

At Anchuruli, where the tunnel emerges, the scale of the work is overwhelming. The perfectly round mouth, taller than three people, opens in the rock beside the lake, and cool, damp air breathes out of it, smelling of stone. Speak and your voice returns from the darkness, doubled and stretched; a dripping stream echoes like a distant drum. In the dry season, a pinpoint of light glimmers nearly four kilometres away. In the monsoon, the silence ends: the Erattayar's floodwater bursts from the tunnel in a roaring white column and crashes into the reservoir below. (Entry into the tunnel is prohibited. Read our guide to Anchuruli.)

Mattupetty and Ponmudi

Mattupetty

High above Munnar, at about 1,700 m, the Mattupetty dam is a concrete gravity dam on the Muthirapuzha river, built between 1949 and 1953 to store water for Kerala's first hydroelectric scheme at Pallivasal. Its long, winding reservoir, framed by tea estates and shola-clad hills, is one of the loveliest sights of the High Range. The perennial water draws elephants, gaur and birds to its shores, and the grasslands nearby were the home of the Indo-Swiss dairy project. Speedboats and pedal boats run on the lake, and the famous Echo Point lies a little further along the road to Kundala, where the hills throw back every shout. (Read Mattupetty and the Indo-Swiss project.)

• Type: concrete gravity dam.
• River: Muthirapuzha, a tributary of the Periyar.
• Built: 1949–1953.
• Purpose: storage for hydroelectric generation downstream.

Ponmudi

About 25 km southeast of Munnar, near Rajakkad, the Ponmudi dam holds back the Panniyar, another tributary of the Periyar, in a quiet valley of spice gardens and small farms. Completed in 1963, it feeds the Panniyar power station. Its reservoir lies in a bowl of hills that seem to answer every sound, and about a kilometre from the dam a slender hanging bridge sways over the river, a crossing that sets hearts racing above the rushing water.

• Type: masonry/concrete gravity dam.
• Height: about 58 to 59 m; length about 290 m.
• River: Panniyar, a tributary of the Periyar.
• Built: completed 1963; the Panniyar power station's first unit ran on 29 December 1963.

Water, power and responsibility

The dams of Idukki generate a large share of Kerala's electricity and irrigate hundreds of thousands of hectares in Tamil Nadu. They have also drowned forests and villages, displaced people, and changed the rivers for ever, and in years of extreme rain, such as 2018, they remind everyone of the power they hold back. To stand beneath the Idukki arch, or beside the old wall at Mullaperiyar, is to feel both the ambition of the engineers and the debt owed to the thousands who built these titans, and to the hills and forests from which their waters flow.